What are "high tide" and "low tide"?

Tidal phenomenon refers to the periodic movement of seawater under the tidal force of celestial bodies (mainly the moon and the sun). Traditionally, the vertical fluctuation of sea surface is called tide, while the horizontal flow of sea water is called tide.

Tide is the first seawater movement phenomenon that attracts people's attention among all ocean phenomena, and it has a very close relationship with human beings. Port engineering, shipping and traffic, military activities, fisheries, salt industry and aquaculture, offshore environmental research and pollution control are all closely related to tidal phenomena. In particular, the endless vertical fluctuation of the sea surface contains huge energy, and the development and utilization of this energy has also aroused people's interest.

1, tidal element

Figure 7- 1 shows the process curve of tidal level fluctuation (i.e. the vertical height of sea surface relative to a certain datum). In the figure, the ordinate is the height of tidal level, and the abscissa is time. When the tide rises, the tide level keeps rising. After reaching a certain height, the tide level does not rise or retreat for a short time, which is called flat tide, and the middle time of flat tide is called high tide. The duration of flat tide varies from place to place, ranging from several minutes to dozens of minutes. After the flat tide, the tide level began to fall. When the tide level recedes to the lowest level, similar to the normal tide level, there is also a phenomenon that the tide level does not retreat or rise, which is called tide stop, and the middle time is low tide. After the tide stopped, the tide level began to rise again, so it moved again and again. The interval from low tide to high tide is called high tide, and the interval from high tide to low tide is called low tide. Generally speaking, in many places, the high tide is not as long as the low tide. The height when the sea surface rises to the highest position is called high tide, the height when it falls to the lowest position is called low tide, and the difference between adjacent high tides and low tides is called tidal range.

2. Tidal inequality and tidal types

I. Types of tides

It can be seen from the tidal observation curves in various places that there are periodic changes in high tide and low tide, high tide and tidal range. According to the period and tidal range of tidal fluctuations, tides can be roughly divided into the following four types:

1. A regular semidiurnal tide has two high tides and two low tides in a lunar day (about 24: 50). The tidal range from high tide to low tide and from low tide to high tide is almost equal. This tide is called regular semidiurnal tide (Figure 7-2a).

2. Irregular semidiurnal tides In most days of a new moon, each lunar day generally has two high tides and two low tides; However, for a few days (when the declination is large in the month), the second high tide is very small and the characteristics of semidiurnal tide are not obvious. This tide is called irregular semidiurnal tide (Figure 7-2b).

3. A normal diurnal tide has only one high tide and one low tide in lunar day. Tides like this are called regular diurnal tides, or regular diurnal tides (Figure 7-2c).

4. Irregular diurnal tide Figure 7-2D is the tidal process curve of irregular diurnal tide. Obviously, this tide has the characteristics of diurnal tide in most days of a new moon, but it has the characteristics of semidiurnal tide in a few days (when the declination of the month is close to zero).

Second, the inequality of tides.

Generally, the tidal range between two tides in a day is not equal, and so is the tide rising and ebbing. This irregular phenomenon is called the diurnal variation of tides. The higher climax is called high climax, and the lower climax is called low climax; Low tide is called low tide and high tide is called high tide.

It can also be seen from the tidal process curve (Figure 7-2) that the tidal range is different every day. The tidal range is the largest in two or three days after the first month of the lunar calendar and the second month of the lunar calendar, and the tidal range at this time is called the tidal range of the spring tide. On the contrary, after the upper and lower chords, the tidal range is the smallest, and the tidal range at this time is called the tidal range of small tides.

References:

Introduction to marine science

Due to the tidal force of the sun and the moon, the lithosphere, hydrosphere and atmosphere of the earth have periodic movements and changes respectively. The elastic-plastic deformation of the solid earth caused by the tidal force of the sun and the moon is called solid tide, which is referred to as solid tide or ground tide for short. The periodic rise and fall, fluctuation and advance and retreat of the sea surface caused by the tidal force of the sun and the moon are called ocean tides, referred to as ocean tides for short; Periodic changes of atmospheric elements (such as pressure field, atmospheric wind field, earth's magnetic field, etc.) (such as 8 12, 24 hours). ) caused by the tide is called the atmospheric tides, referred to as the tide. Among them, the atmospheric tide caused by the sun is called the solar tide, and the lunar tide is called the lunar tide. Because the moon is closer to the earth than the sun, the ratio of tidal force between the moon and the sun is 1 1: 5. For the ocean, the moon tide is more important than the sun tide. The driving forces of earth tides, ocean tides and atmospheric tides are all caused by the different gravitational forces of the sun and the moon on various parts of the earth, and they influence each other. The elastic-plastic tidal deformation of the ocean bottom crust will cause corresponding tides, that is, tides have tidal effects; The migration of seawater quality caused by tidal waves changes the load borne by the crust and makes it bend again. The air tide is above the sea tide, which acts on the sea surface and causes additional vibration, making the change of the sea tide more complicated. As a complete tidal science, its research object should be solid tide, ocean tide and atmospheric tide as a unified whole. However, because the tide phenomenon is very obvious, and it is closely related to people's life, economic activities, transportation and so on, it is customary to understand the word tide in a narrow sense.

Tidal energy is ocean energy in the form of potential energy, which refers to the potential energy of water formed by seawater fluctuation. The tidal phenomenon of seawater fluctuation is caused by the movement and interaction between the earth and celestial bodies. In the ocean, the gravity of the moon raises the water level of the earth facing the moon and the surface facing the moon. Due to the earth's rotation, this water level rise propagates from east to west in the form of deep-sea waves, with a period of 12 hours and 25 minutes and an amplitude less than1m. The effect of solar gravity is similar, but the force is smaller, and its period is 12 hours. When the sun, the moon and the earth are in a straight line, there is a spring tide; When they are at right angles, a small tide will be generated. In addition to the semidiurnal tide and the monthly periodic tide, the rotation of the earth and the moon also produces many other periodic cycles, which range from a few days to several years. At the same time, the seawater on the surface is affected by the centrifugal force of the earth's motion, and the resultant force of the lunar gravity and centrifugal force is the tidal force that causes the seawater to fluctuate. Besides the moon and the sun, other celestial bodies will also generate tidal force on the earth. Although the mass of the sun is much larger than that of the moon, the distance between the sun and the earth is also much larger than that between the moon and the earth, so its tidal force is less than half that of the moon. Because other celestial bodies are far away from the earth or their mass is too small, the tidal force can be ignored. If calculated by gravity, the maximum tidal force generated by the moon can raise the sea surface by 0.563 m, and the role of solar tidal force is 0.246 m, but the actual tidal range is much larger than the above calculated value. For example, the maximum tidal range of Hangzhou Bay in China is 8.93 m, and the maximum tidal range of Fendi Bay in North America and Canada is19.6m. At present, there is no exact explanation for this difference between reality and calculation. It is generally believed that when ocean tidal waves impact the continental shelf and coastline, the tidal range will increase through the movement of rising, gathering and vibration. The energy of tidal energy is directly proportional to tidal volume and tidal range. In other words, it is proportional to the square of tidal range and the area of reservoir. Compared with hydropower, the energy density of tidal energy is very low, which is equivalent to the level of micro-head power generation. The large tidal range in the world is about 13 ~ 15m, but generally speaking, the average tidal range above 3m has practical application value.

Tides vary from place to place, and different regions often have different tidal systems. They all get energy from deep-sea tidal waves, but they have their own characteristics. Although the tides are complex, they can be accurately predicted anywhere. Ocean tides get energy from the earth's rotation and slow it down in the process of absorbing energy. However, this deceleration of the earth's rotation is almost imperceptible in a person's life and will not be accelerated by the development and utilization of tidal energy. This energy is dissipated at the rate of 1.7TW through the friction between the shallow sea area and the coastal area. Only where there is a spring tide, energy concentration and geographical conditions are suitable for building tidal power stations can energy be extracted from the tides. Although such places are not everywhere, countries all over the world have chosen a considerable number of sites suitable for developing tidal energy. According to the latest estimate, the tidal energy with development potential is about 200 MWh per year.

The theoretical reserve of tidal energy in the world is about 3 × 109kw. China's coastline is tortuous, with a total length of about 1.8× 104km, and there are more than 6,000 large and small islands along the coast, forming a coastline of 1.4× 104km. The long coastline contains abundant tidal energy resources. The theoretical reserves of tidal energy in China are 1. 1× 108kw, of which Zhejiang and Fujian provinces have the largest reserves, accounting for about 80.9% of the whole country, but these are theoretical estimates, and the actual available ones are far less than the above figures.